Target allocator support for other receivers than Prometheus

[decimalst]

Is your feature request related to a problem? Please describe.
Hey folks, I am reviewing the documentation for scaling the Opentelemetry collector here.
In my use case, we have an arbitrary number of prometheus metrics to scrape, so the target allocator is useful since we can horizontally scale to meet our load. However, I was hoping we could use the same tool for other scraping receivers, such as Redis or databases that require authentication.

Describe the solution you'd like
Adding support for other receivers in target allocator.

Describe alternatives you've considered
I took a look at how the Prometheus target allocator works, and it seems it relies on Prometheus's HTTP service discovery feature to function. I am not sure if the right path is to add a service discovery method to other receivers. It would be nice if something similar could be accomplished with a centralized service that does some sort of "pass through" of connection requests to a list of targets without needing to modify every receiver we might want to support service discovery. Maybe I’m just describing a service gateway?